Sleazy bar outside City area is influential (8)
I believe the answer is:
decisive
'influential' is the definition.
(similar in meaning)
'sleazy bar outside city area is' is the wordplay.
'sleazy bar' becomes 'dive' (dive can mean a seedy or sleazy bar).
'outside' is an insertion indicator (some letters go outside others).
'city area' becomes 'EC' (EC is a London postcode area).
'ec'+'is'='ecis'
'dive' enclosing 'ecis' is 'DECISIVE'.
(Other definitions for decisive that I've seen before include "Resolute" , "Settling an issue quickly" , "Producing a definite result" , "Showing firmness, making mind up without dithering" , "Crucial" .)